1. Field of the Invention
The present invention relates to a nitride-based semiconductor laser device and a method of fabricating the same, and more particularly, it relates to a nitride-based semiconductor laser device having a facet and a method of fabricating the same.
2. Description of the Prior Art
A nitride-based semiconductor laser device is recently expected as a light source for a future large capacity disk, and actively developed. The recording speed of a recording optical disk system must be increased due to the rapid enlargement of the market for the recording optical disk system represented by a CD-ROM. In order to increase the recording speed of the recording optical disk system, the light output power of the nitride-based semiconductor laser device must inevitably be increased. Thus, the technique for increasing the light output power of the nitride-based semiconductor laser is extremely important.
FIG. 20 is a perspective view showing a conventional nitride-based semiconductor laser device. Referring to FIG. 20, an n-type contact layer 102 of n-type GaN having a thickness of about 5 μm, an n-type cladding layer 103 of n-type AlGaN having a thickness of about 1 μm and an active layer 104 having a thickness of about 0.1 μm are formed on a sapphire substrate 101 in the conventional nitride-based semiconductor laser device. A p-type cladding layer 105 of p-type AlGaN having a projection is formed on the active layer 104. A p-type contact layer 106 of p-type GaN is formed on the projection of the p-type cladding layer 105. The projection of the p-type cladding layer 105 and the p-type contact layer 106 form a ridge portion. A p-side ohmic electrode 107 having a thickness of about 0.5 μm is formed on the p-type contact layer 106 to be in contact with the overall upper surface of the p-type contact layer 106.
The layers from the p-type cladding layer 105 to the n-type contact layer 102 are partially removed. Current blocking layers 108 of SiO2 are formed to cover part of the exposed upper surface of the n-type contact layer 102, exposed side surfaces of the n-type contact layer 102, the n-type cladding layer 103, the active layer 104 and the p-type cladding layer 105, the upper surface of the p-type cladding layer 105 and the side surfaces of the ridge portion. A p-side pad electrode 109 having a thickness of about 1 μm is formed on the current blocking layers 108 to be in contact with the p-side ohmic electrode 107 formed on the upper surface of the ridge portion.
An n-side ohmic electrode 110 is formed on a surface part of the n-type contact layer 102 exposed by partial removal. An n-side pad electrode 111 is formed on the n-side ohmic electrode 110.
In the conventional nitride-based semiconductor laser device having the aforementioned structure, a current flows from the p-side pad electrode 109 to the active layer 104, the n-type cladding layer 103, the n-type contact layer 102, the n-side ohmic electrode 110 and the n-side pad electrode 111 through the p-side ohmic electrode 107 as well as the p-type contact layer 106 and the p-type cladding layer 105 forming the ridge portion. Thus, a laser beam can be emitted in a region of the active layer 104 located under the ridge portion.
The aforementioned conventional nitride-based semiconductor laser device is formed by cutting the wafer prepared by forming the aforementioned layers and electrodes 102 to 111 on the sapphire substrate 101 into a prescribed size. In this case, a facet of the nitride-based semiconductor laser device cut from the wafer is formed by cleavage or dry etching for emitting laser beam. The facet functions as a reflector of a cavity.
When the facet of the conventional nitride-based semiconductor laser device is formed by cleavage, the facet is readily damaged due to stronger mechanical strength of the nitride-based semiconductor as compared with other compounds such as GaAs and GaInP. Particularly in the conventional nitride-based semiconductor laser device shown in FIG. 20 having the layers 102 to 106 consisting of nitride-based semiconductors formed on the sapphire substrate 101, the crystal axes of sapphire and the nitride-based semiconductors mismatch with each other and hence readily cleavable directions of the sapphire substrate 101 and the nitride-based semiconductors also mismatch with each other. Therefore, the facet of the conventional nitride-based semiconductor laser device shown in FIG. 20 is readily damaged when the same is formed by cleavage. In this case, the facet formed by cleavage may be uneven.
In order to substantially vertically form a facet of a nitride-based semiconductor laser device by dry etching such as RIE (reactive ion etching), a physical etching element must be strengthened. When the physical etching element is strengthened, however, the facet formed by etching is readily damaged. In order to form a facet of a conventional nitride-based semiconductor laser device by etching, an etching mask is generally prepared from SiO2. An end face of this etching mask of SiO2 is uneven in a striped manner in etching. In this case, the facet of the nitride-based semiconductor laser device formed by dry etching reflects the shape of the etching mask. When the facet of the nitride-based semiconductor laser device is formed through the etching mask of SiO2, therefore, the facet of the nitride-based semiconductor laser device is also uneven in a striped manner.
As hereinabove described, the facet of the conventional nitride-based semiconductor laser device is remarkably damaged and uneven dissimilarly to a GaAs- or AlGaInP-based semiconductor laser device. Thus, the n-type cladding layer 103, the active layer 104 and the p-type cladding layer 105 on the facet are remarkably damaged and uneven in the conventional nitride-based semiconductor laser device, to increase the number of crystal defects in the vicinity of the facet. When a current is injected into the conventional nitride-based semiconductor laser device, therefore, a non-radiative recombination current emitting no light flows in the vicinity of the facet. This non-radiative recombination current increases the temperature in the vicinity of the facet and absorbs the laser beam. Thus, the band gap in the vicinity of the facet is reduced to further increase absorption of the laser beam in the vicinity of the facet. The conventional nitride-based semiconductor laser device is disadvantageously deteriorated in the vicinity of the facet by repeating the aforementioned process.